System of carrier current transmission



May 2, 1939. J. M. D. DEKKER SYSTEM OF CARRIER CURRENT TRANSMISSION Filed April 25, 1956 2 Sheets-Sheet l .ukSv mm om.

May 2, 1939. J* M. D. VDEKKER SYSTEM OF CARRIER CURRENT TRANSMISSION Filed April 25, 1936 2 Sheets-Sheet 2 Btl/unsub M. m, pw

5 y 5 N W 7 l A A Patented May 2, 1939 i UNITED STATES PATENT olll-lcl-z SYSTEM F CARRIER CURRENT TRANSMISSION Jan Maurits Douwes Dekker, The Hague, Netherlands, assigner to Associated Electric Laboratories, Inc., Chicago, Ill., acorporation of Delaware Application April 25, 193s, serial No. 76,382 In the Netherlands April 30, 1935 9 Claims. (Cl. 179-15) 'I'his invention relates to improvements in carfrequencies up to twice the normal maximum rier-current communication systems, and more value, without changes in the filters. particularly to systems for carrier-current trans- According to the invention the channels are mission in cables. grouped in pairs having common transmitting In carrier-current transmission in cables it is circuits when a passage is made through the desirable to locate a maximum number of chancable or other transmitting medium, the channels in a given frequency range in order that nels of each pair being so arranged that the low the system may provide as many separate transmodulation frequencies of both lie at the limits mission channels as possible. of the pass band of the transmitting circuit, and

l0 To elect this desired result, sharpv band filters each group of two channels is separated from the 10 are a requirement; moreover, these filters have adjacent groups by a single band filter. The to be of relatively small band-pass width, in pardistance between the carrier frequencies is made ticular for the higher frequencies. The lattery such that they substantially coincide with the requirement necessitates Iaccurate tuning of the limiting frequencies of the pass bands. elements and very low power factors, with the In practice it is particularly advantageous to attendant design and economic difculties, locate the unused signal sidebands in the unused By way of example, the frequency scale which portions of the total available frequency range, Was used in the Morristown experiment (seesthe e eendtien Which may be realized by making Bell system Technical Journal for July 1933, the unused -or attenuating portions of the range a pages 251-263) may be considered. The sepa.- su'iciently wide. By applying the present inven- 20 ration of the carrier frequencies in this case was tien. Such all arrangement iS POSSible WithOut 4000 Cycles, and a band 2500 cycles wide was consequent diminution of the useful percentage transmitted by each channel, the modulation- 0f the frequency range because of the double frequency band extending from 250 to 2750 cycles. Width 0f the pass and Step bandS- e 5 'I'he single (lower) sideband method of trans- At the receiving end 0f the Cable 0r Other 25 mission was used, the carrier being suppressed. transmitting medium the tWO Sidebalids in each If the theoretical band-pass width of each nlter pair of channels are supplied to two separate is assumed to be 2700 cycles, a, Space 1300 cycles demodulators, each of which is supplied with one wide remains between pass bands, which is less of the two corresponding carriers. I 3o than 50 percent of the width of a transmission If care is taken to make each carrier sufi- 30 band.i The attenuation within these spaces ciently strong relatively to any residue of the should be about 60 decibels, if the noise in a' otherl carrier which may be Present, the 10W- channel due to an adjacent channel is to be kept pass filter following each demodulator will arrest withinbounds. (The noise resulting from such the undesired response coming from the other inter-channel interference is not correctly re-l channel of the pair. 35 .ferrea to as cross talkpecause such noise is 'I'he principle of the invention, which consists unintelligible.) In the Morristown system, it was in pairing off the speech channels in the manpossible to ldivide the frequency-range between 0 .ner indicated (with the higher modulation freand 39-ki1ocycles into 10 separate channels,y there- Y quencies adjacent each other). may also be ap-r by permittingvthe simultaneous transmission of 10 plied t0 a ,System in Which modulation and de* 40" com/ersgsi-,10mgA 'l :f: u f s .modulation are effected in two steps in the man- Itis an objectI ofthe presentinventionto 'p'ro ner of a superheterodyne receiver. In this case I vide a vcarrierecurrent transmission system which the transmission circuit for each Set of two chan'- 'nels cnsistsof an auxiliary modulator with aky l requiresfa roximatel half -.the number'of `filters l pp y band filter connected to .its input, this auxiliary"r 45 y which are normally employed, without dirniriufv tion of .the useful Vpercentage of the frequency range.,V .m5, .r n f y Another object "of the invention vis to vprovide frequency range of the transmitting medium.

a carrierg-current L transmission system inwhich @The sidebands supplied to .the'input'fband filter 50 are derived froma pair of modulators, each one.

thefilter requirements are much-f less stringent than inpreviously-known systems. l f' `of which islsupplied'with one or the `nflodulation- An additionalobject Xiof the invention` is to' rfIeCll-leny Signals* and'lwibh' iteeliel having "-8,

provide-:aucarrienfcurrentp-transmission systemk frequency `situated near a lcut-off frequency "modulator transferring the set of two sidebands supplied --to ythe filter toY a'givenpoint in the whichgrnay @be'adaptedto transmit modulationifthe band filter, the zvfc-:arrierc frequencies' `'axidt'hen band-filter characteristic being the same for all pairs of channels.

In systems of the type contemplated herein, a receiver may be used in which the currents from the transmitting circuit areconducted along two diiierent transmission paths, each of which its supplied with one of the corresponding carriers, and each of which includes a demodulator followedv by low-frequency selective transmitting means.

The invention will be better understood by refference to the accompanying drawing, in which:

Fig. 1 is a diagrammatic representation of a conventional form of carrier-current transmission system;

`Fig. 2 is a diagrammatic representation of a carrier-current transmission system in accordance with my invention;

Fig. 3 is a block diagram of a carrier-current transmission system in accordancewith my invention;

Fig. 4 shows graphically the performance of a systemaccording to Fig. 3; and

by -the curves l.

location of the carriers in the frequency range..

tical arrows, are spaced at 400G-cycle intervals. f

'I'he lowest channel carries ordinary modulation frequencies and therefore no corresponding carrier is shown. The representation of this gure may be taken as typical of known carrier-current transmission systems.'

Fig. 2 4represents the frequency range oiV a transmitting medium, extending from 0 to about 40 kilocycles. As shown, this region comprises 9 channels, 8 of which are combined in pairs, in accordance with the invention, in such' a manner that their high modulation frequencies are substantially' adjacent, the lower limits of the audio-frequency region lying near the cut-o5 frequencies of the respective transmitting circuits, which Vare connected to the transmitting medium...l The transmitting characteristics of these circuits are schematically'shown in Fig. 2 The arrows vi. indicate the yAs will be evident upon comparison with Fig. 1, the system according to the invention provides a saving of` about 50' percent in the number of filters required. Moreover, the suppression bands are twice as wide in the new system as inthe old; this permits locating the sidebands not desj tined for transmission entirely in the suppression bands, while maintaining the same useful per- Y- centage o f the entire transmitting range.

sender andreceiver band filters arefmade.` to

lcontribute', toward the suppression'of 4undesired gtransmission'and therefore of lnoisedor so-called cross talk. 4 v

The carriers may be suppressed more -or less completely by the use of balanced modulators;

the attenuation near Vthe edges of the pass bands may also contribute to'this suppression. The functioning'of the system is not dependent on the strength ofthe transmitted carriers.

y BYl these meansfthe' suppression fbands, of both thel 'Ihe arrangement of apparatus for a system functioning according to the scale division of Fig. 2 may be as shown symbolically in Fig, 3. At the transmitting end of line 3, modulators 2| and 22 are connected through band pass filter Y 23 tothe line. Modulators 2| and 22 are supplied from audio-frequency sources 24 and 25, respectively, and additionally have supplied to them carriers I and II, respectively, as shown.

Modulators 2i and 22 are arranged to suppress lli the carriers. At the receiving end of line 3, band filter 4 is shown connected to the line. Band filters 4 and 23 may, for instance, have the transmitting properties shown graphically in Fig. 2at l, a pass band being .shown at a, -and a 15 suppression band at b. The sidebands passed,u by filter 4 are supplied to a pair of parallel transmission paths 5 and 6, which include demodulators 1 and 8, respectively. ,One of the -respec- 'tive carriers from a local source at the receiving end .is supplied to one of these demodulators, the other carrierbeing fed to the other demodulator, a requirement being that the amplitudes ofthese carriers be sufciently large with respect to any residual carrier which may have 25 come through the band filter to minimize cross talk. If this requirement is satisfied, the signal or modulation transmitted on one channel is obtained from the demodulator 1, and the signal or modulation of the other channel passed by 3o the band filter 4 is obtained from the demodulator 8. Low-pass filters 9 and I0 connected to the demodulator outputs transmit only the desired audio-frequency range and suppress all undesired frequencies above this range.

The'operation of the arrangement of Fig. 3 is illustrated in the graphs shown" in Fig. 4. Fig. 4a shows the frequencies which are passed by the-band, filters 4 and 23. Block I represents the upper sideband transmitted by carrier I, and block II corresponds to the lower sideband of carrier II. Carriers I and II are not transmitted, and hence are indicated by broken lines. Fig. 4b

- shows the voice or audio frequencies which appear at the output of low-pass lter 9, corresponding with theupper sideband of carrier I 45 'with'the -lower sideband of carrier II and represented by block II. Since these frequencies cover the same range as those represented by block -I of Fig. 4b, block II appears in the corresponding position in Fig. 4c. The audio frequencies which correspond with the upper sideband of carrier I are very weak at the output of demodulator 8 and hence do not appear at the output of filter I0. Therefore they are represented by'dotted block I. The curve Il of Fig. 4c indicates the operating characteristics of lters 9 and l0. y l y Fig. 5 illustrates the application of the principle of the invention to a system in which modulation and demodulation lare effected in two stages, analogous manner to the operation 10 vof a. superheterodyne receiver.

- As shown at the top in Fig. 5 a separate first modulator is provided for each channel. In order to obtain pairs of two adjacent sidebands the signals of one of the channels. which shall make up such a pair, are modulated with the carrier do in the iirst modulator I2, the other channel being modulated with the carrier do' in the second first modulator I3. The carriers do and du' are situated respectively near the upper and lower pass-band limits of the band filter shown in the right-hand upper portion of Fig. 5, curve I4 being the attenuation characteristic of this filter.

To every set of two channels is assigned a set of modulators I2 and I3 and a band filter with the characteristic I4, the frequencies do and do' as well as the characteristic I4 being identical for all the pairs; the corresponding band filters are therefore also identical for all pairs.

A separate auxiliary modulator I5 is provided for each pair of channels, the sidebands passed by the band filter being modulated in this modulator with an auxiliary carrier dn, which has a different frequency for each set of two channels and which is suppressed by the modulator. The outputs of the auxiliary modulators I5 are connected'to the line or other transmitting medium. As a result of the arrangement described the pairs of channels will occupy the correct relative positions in the frequency scale of the transmitting medium; the steps leading 'up to this result are shown in the lowest graph of the upper half of Fig. 5, as well as the resulting positions of the channels.

At the receiving end the transmission medium or line is provided with as many auxiliary modulators as there are pairs of channels. iliary modulator I6 is supplied with the auxiliary carrier du and thus transposes one pair of sidebands within the pass band of a band filter Il. These filters are-present in a number equal to the number -of the pairs of sidebands. After having been demodulated to eliminate the auxiliary carrier du, the nth and the (1L-|- 1)th sidebands are-passed to the input of band filter I1. In the demodulators I8 and I9 the received currents from the output of the filter II are demodulated to eliminate the carriers do and do'.

. In the outputs of these demodulators low-pass audio-frequency filters are provided, the oper-v ating characteristics of which are shown in Fig. 5 at 20. The operation is such that the nth channel is obtained from one filter and the (n+1)th channel from the other filter.

'I'he frequencies of carriers do and do are preferably of such a value that they lie above the frequency range to be transmitted, as well as their sidebands and the carrier dn; if this is done there is no need for any further separate filterchannels it is desirable: first, to work with car' rier suppression in the sending modulator; and second, that the filters effect a maximum attenuation of the carriers. T he frequencies limiting the pass band of the filter should therefore lie as close tothe frequency band to be transmitted as possible, in., order to obtain positive attenuation of the carrier. As, moreover, the

,frequency bands of a pair of channels are juxtaposed in such a manner that the portions conl taining the harmonics of speech (which represent The aux-l very little energy) are near each other, these bands may be approached quite closely without entailing serious diiliculties in audio-frequency filtering.

The pass band of each filter is thereby made 'about twice as wide as in the known system. If

quirements of the filters become easier of realization.

Anv additional advantage of the system is that if occasion arises a musical program may be transmitted in lieu of a pair of channels, the networks being retained unchanged.

Having thus described my invention, what I claim is: 1. A carrier transmission system including plural sources of modulation; plural sources of carriers, said carriers being alternately spaced in frequency; modulators for combining each of said modulations with one of said carriers to produce plural modulated carriers each having upper and lower sidebands; transmitting filters each connected to frequency-adjacent pairs of said modulators, said filters being adapted to suppress. the carriers, the lower sideband of the lower-frequency carrier,` and the upper sideband of the higher-frequency carrier in each of said pairs; and unitary means for simultaneously transmitting the remaining pairs of upper and lower sidebands to a receiving station.

2. A carrier transmission system including plural sources of modulation; plural sources of carriers, said carriers being alternately equally spaced in frequency; modulators for combining each of said modulations with one of said carriers to produce plural modulated carriers each having upper and lower sidebands; transmitting filters each connected to frequency-adjacent pairs of said modulators, said filters being adapted to suppress the carriers, the lower sideband of the lower-frequency carrier, andthe upper sideband of the higher-frequency carrier vin each of said pairs; and unitary means for, simultaneously transmitting theremaining pairs of upper vand lower sidebands to a receiving station.

3. A carrier transmission system including plural sources of modulation; plural sources of carriers, said carriers being alternately spaced in frequency; modulators for combining each of said modulations with one of said carriers to produce plural modulated carriers each having upper and lower sidebands; transmitting filters each connected to frequency-adjacent pairs of said modulators, said filters being adapted to suppress the carriers, the lower sideband of the lower-frequency carrier, and the upper sideband of the higher-frequency carrier in each of said pairs;

unitary means for simultaneously transmitting the remaining pairs of upper and 'lower sidebands to a receiving station; receiving filters at said receiving station each adapted to select one of said transmitted pairs of sidebands; two demodulators associated with each of said receiving filters; means for supplying to each of said demodulators a current corresponding in frequency to one of the suppressed carriers; and low-pass filters associated with the outputs of each of said demodulators arranged to transmit only the frequencies of the original modulations.

4. The method ofconducting plural communicationsin a single carrier transmission medium which comprises impressing said communications separately upon carriers alternately spaced in frequency vthroughout a frequency range, sup-y pressing the carriers, suppressing alternate upper and lower sideband frequencies, and transmitting the remaining lower and upper side-band frequencies through saidmedium to a receiving station, the spacing of said carriers exceeding the frequency band occupied by each pair of said remaining sideband frequencies. t

5. I'he method of conducting plural communications in a single carrier transmission medium which comprises impressing said communications separately upon carriers alternately equally spaced in frequency throughout a frequency range, suppressing the carriers, suppressing a1- ternate upper and lower sideband frequencies, and transmitting the remaining lower and upper sideband frequencies through said medium to a receiving station, the spacing of said carriers exceeding -the frequency band occupied by each pair of said remaining sideband frequencies.

6. The method of conducting plural communications in a single carrier transmission medium which comprises impressing said communications separately upon carriers alternately spaced in frequency to produce modulation of said carriers with resulting upper and lower sidebands, suppressing said carriers and alternate upper and lower sidebands so that the remaining alternate lower and upper sidebands constitute adjacent pairs of sidebands, the frequency limits of each pair corresponding to the lower modulation frequencies, and transmitting said pairs of sidebands through said medium to a receiving stationjthe spacing of said carriers exceeding the frequency band occupied by each of said pairs of sidebands.

' 7. The method ofV conducting plural communications in a single carrier transmission medium which comprises impressing said communications separately upon carriers alternately equally spaced in frequency to produce modulation of said carriers with resulting upper and lower sidebands, suppressing said carriers and `alternate upper and lower sidebands so that the remaining alternate' lower and 'upper sidebands constitute adjacent pairs of sidebands, the frequency limits of each pair corresponding to the lower modulation frequencies, and transmitting said pairs of sidebands through said medium to va receiving station, the spacing of said carriers exceeding the frequency band occupied by each of said pairs of sidebands.

8. I'he method of conducting plural communications in a single carrier transmission medium which comprises impressing said communications separately upon carriers alternately spaced in frequency throughout ra frequency band, suppressing the carrier frequencies,suppressing alternate upper and lower sideband frequencies, trans-A mitting the remaining lower and upper sideband frequencies through said medium to a receiving station, removing any remaining residue of the suppressed upper and lower sideband frequencies, separating said remaining lower and upper sideband frequencies into pairs each consisting of an upper sideband and an adjacent higher-frequency lower sideband, supplying the previously suppressed carrier frequencies separately to each of said pairs, and demodulating each carrier and its -associated sideband frequencies to reproduce said communications.

9. The method of conducting plural communications in a single carrier transmission medium which comprises impressing said communications separately upon -carriers alternately spaced in frequency to produce modulation of said carriers with resulting upper and lower sidebands, suppressing said carriers and alternate upper and lower sidebands so that the remaining alternate lower and upper sidebands constitute adjacent pairs of sidebands, the frequency limits of each pair corresponding to the lower modulation frequencies, transmitting said pairs of sidebands through said medium, conducting each of said pairs separately to a pair of demodulators, supplying the previously suppressed carriers separately to said demodulators, and substantially suppressing all frequencies in the outputs of said demodulators except the modulation frequencies corresponding to said plural communications.

JAN MAURITS DOUWES DEKKER. 

